The present invention relates to electronically controlled pressure systems. It finds particular application in conjunction with control models for controlling pressure in pneumatically operated vehicles systems and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.
Electronic controlled brake systems (ECBS or EBS), antilock braking systems (ABS), and automatic traction control (ATC) systems are incorporated into vehicles to improve braking performance and vehicle handling. EBS permits perpetual optimal balancing of braking forces among individual wheel brakes and for achieving optimal stability and braking performance during all driving and braking situations. Proportional-solenoid or multiple-solenoid controlled valves are used for distributing and modulating desired pressure to the individual wheels as processed and controlled by an electronic control unit (ECU). The ECU receives sensor input signals from, for example, the driver's brake pedal demand, the speed of individual wheel(s), along with control and brake chamber pressures.
The brake pressures calculated by the ECU for the individual wheels must be delivered to the respective wheels with a high accuracy and, furthermore, must be delivered and adjusted very quickly. Therefore, pressure modulator relay valves are used in air-braked systems to achieve quick pressure apply and release times. Also, additional pressure sensors are used to achieve desired pressure accuracies.
A modular relay valve (MRV) operates as a remote controlled brake valve for delivering or releasing air to brake chambers in response to control air that is delivered from the driver's operated brake valve or other sources. The relay valve applies, holds, or releases a brake chamber's pressure in proportion to the control pressure, which is controlled as a function of the driver's brake pedal demand.
ABS and ATC as integrated in EBS prevents wheel lock-ups during braking and excessive wheel spinning during accelerating in order to provide vehicle stability and braking and traction performance.
MRVs used in conjunction with EBS typically include three (3) solenoids for controlling the air pressure. A backup solenoid (electrically) provides supply pressures from an air reservoir; a hold solenoid maintains air pressure; and a release solenoid removes or exhausts air pressure. An MRV used in conjunction with EBS may only include a single solenoid, which is designed as a pressure/current proportional solenoid.
A proportional solenoid converts a control current, which is determined by an algorithm in the ECU, into a proportional control pressure for the relay valve. One advantage of controlling pressure with a proportional solenoid is the possibility of providing and controlling different pressure curves and pressure modulations as a function of the control current supplied to the solenoid. However, proportional solenoids are more complex and expensive and, furthermore, require the ECU to supply a current controlled output stage that acts as the control current.